A young horse chestnut tree in a west London garden was bleeding a rust-coloured liquid down its bark last week. As it dried, it left a black, tar-like stain several feet long. Two streets away, another horse chestnut had prematurely shed its leaves in July. A fungal pathogen known as bleeding canker was killing the first tree from the inside, while the horse chestnut leaf miner moth was destroying the second. Neither tree will survive the winter.

Meanwhile, in Plymouth last week, parts of a mature cypress hedge were turning a lighter olive-grey than similar trees around it. It was a sure but ominous sign that a fungus-like pathogen known as Phytophthora lateralis was attacking its roots and working its way up the inner bark. The disease has left a swath of destruction in the western states of the US and Canada, but its arrival in England for the first time has struck fear in gardeners' hearts.

In scenes reminiscent of foot and mouth disease 10 years ago, the trees were quickly felled, and their roots burnt because the infection can spread through soil.

Plant pathogens are on the rise globally and Britain is susceptible because of our increasingly warm, wet winters and the globalisation of trade, which has enabled us to fuel our love of gardens by importing millions of exotic plants, many of which can arrive diseased.

"We now have six to eight organisms in the British Isles that are a real concern. In the 1960s and 70s it was Dutch elm disease, which killed 30m trees; in the 1990s it was a new Phytophthora which devastated alders along river banks. But in the last 10 years we have had as many new diseases as we had in the previous 40 or 50 years," said Joan Webber, principal pathologist at Forest Research, the Forestry Commission's research arm.

More than 3m larches have had to be felled in the last three years to try to stop the spread of an airborne disease called Phytophthora ramorum. Meanwhile, hundreds of cases of a bacterial infection, acute oak decline, have been recorded in mature oaks; a disease called red band needle blight is affecting many conifers; and beeches and ash trees have all come under sustained attack by exotic pests, fungi and diseases. Most are confined to one species of tree, but they can potentially jump species into others.

Tony Kirkham, head of the arboretum at the Royal Botanic Gardens, Kew, which has 14,000 trees and has seen many attacked in the last few years, said some of the most serious threats came from the oak processionary moth. These have been seen in Britain before but were first found breeding in 2005 along a stretch of the A40 and in Kew and East Sheen, west London. The caterpillars can cause serious defoliation of oak trees, and weaken the trees to the point that they are prone to other diseases.

"It came over in nursery stock from the continent. We think we have cornered it it in south-east England but it spreads on the air and it could devastate our trees. Now the similar pine processionary moth has reached Paris from Italy and Spain and is marching across Europe towards us. It is only a matter of time before it arrives. We are really in the frontline," said Kirkham.

"This is serious stuff. It will change the landscape of the British Isles if it is not checked.

"It's a very challenging time for anyone who manages trees. Bleeding canker is spreading right across southern England, and is now moving north quickly, as far as Wales and York. Wherever you go in southern England, horse chestnuts are struggling," he said.

Roger Coppock, head of analysts at the Forestry Commission, said: "The international trade in plants has doubled in the last 10-15 years. A lot of plants are coming in from the EU, but are originating from much further afield. There has been huge demand for European plants. Our obsession with gardening is partly the problem. If we don't get these threats under control, the wider long-term social, environmental and economic impacts will be very significant indeed."

The nightmare for foresters would be a major pest attacking sitka spruce, which would devastate the commercial wood industry, but there are fears that other diseases not yet in Britain – but getting closer all the time – could quickly wipe out ash, pines, rhododendrons or other plants.

The most likely route into Britain, says the Forestry Commission, is via semi-mature plants arriving diseased in garden centres. There are plant passports and checks but these must be tightened up, said Coppock.

Environment secretary Caroline Spelman last week made £7m available for further research. "If we don't act now we could end up in a similar situation to the 1970s, when more than 30 million trees in the UK died due to Dutch elm disease," she said.

The new diseases are largely here to stay, and it's a case of adapting to the "new normal", say foresters. "Seven million is a start, but we will need more to hold off these new diseases. This is one of the most serious threats we have seen to Britain in many years," said Kirkham.